X-ray analysis apparatus



Feb. 9, 1960 2 Sheets-Sheet 1 Filed June 1, 1956 27 ELECTRICAL OUTPUT X-RAY OPTICS REGION CRYSTAL ME E m WWE%M V E N 7 mm W HF. F. L 2 3% LM RA 5 MS M u m 7 m F U U 0 H U a m o m m m %m a a m 1 m m w a 32 PUMPING I MEANS I Fig. 2

Feb. 9, 1960 c, HENDEE ErA L 2,924,715

X-RAY ANALYSIS APPARATUS Filed June 1, 1956 2 Shams-sheet 2 D Z 2 9 Ll. m

(9 E oRoN(z=5) 0 U o l I Q I I I PRESSURE mm.) OF HELIUM GAS Fig. 3

ULTRA-SOFT X-RADIATION SOURCE DETECTION i PRESSURE CONTROL REGION N6 MEANS INVENTORS CHARLES F. HENDEE\ GA suP LY F 1g, 4 BY SAMUEL FINE AGENT United States Patent 2,924,715 X-RAY ANALYSIS APPARATUS Charles F. Hendee, Hartsdale, and Samuel Fine, New York, N.Y'., a's signors to North American Philips Company, Inc, New York, N.Y'., a corporation of Delaware Application June 1, 1956, SerialNo. 588,898

Claims. c1. 250*:53

This invention relates to a method and system for the chemical analysis of materials by means of X-radiation.

Our copending application, Serial No. 562,412, now Patent No. 2,837,656, filed January 31, 1956, describes an X-ray analysis system for analyzing the fluorescent X- radiation of elements in the third and fourth periods of the periodic table. In this system, a very low absorption gas, for example, helium, is employed in the X-ray path to minimize any reduction in intensity of the X-radiation as it traverses said path, whereas a higher absorption gas, for example argon, is employed in a detector to ensure that any radiation entering the detector is fully absorbed therein and thus productive of electrical information. This requires some form of partition or separation means to isolatethe X-ray generating and detector regions containing the different gases, which partition had the form'of a very thin window of, for example, beryllium, which was gas-permeable. Employing such an extremely thin Window was essential in order to enable this fiuorescent X-radiation, which is commonly known as sof X-radiation when it is characteristic of the elements in the third and fourth periods of the periodic table, i.e., the K-radiations' of these elements, which lie in the wavelength range of about 12. to 2 Angstroms, to enter the detector. Operation of the system thus described is excellent for the range of elements in the third and fourth periods. However, attempts to extend the analysis capabilities of the system to elements in the second period ofthe periodic table were found impossible. These elements include lithium up to fluorine, and even possibly several of theelements in the early part of the third period, which elements when excited produce fluorescent X-radiation with wavelengths between about 200 and 12 Angstroms, commonly known as ultra-soft X-radiation. The advantages of being able to analyze elements in the second period are evident when it is realized, for instance, that all organic materials contain carbon, and in addition frequently contain large amounts of nitrogen and oxygen, all elements lying in this second period. The impossibility of making a system as described in our copending application operable with second period elements is due to the fact that the thinnest possible window presently feasible still exhibits so little transmission for the ultra-soft X-radiation that essentially none of said radiation is able to enter the detector and produce a usable response.

The primary object of the present invention is to provide a system and method for chemical analysis of materials by means of X-radiation and'which is capable of analyzing the fluorescent X-r'adiation produced by elements in the second period of the periodictable.

The present invention is based upon a wholly new approach to the problem; Briefly, the system or apparatus of the invention comprises an enclosure having a fir'st regionin whicHultra-so'ft X radiation is generated and, if desired, monochromati'zed, collimated, or both, and a second region in which the ultra-soft X-radiation is received and detected to produce in an output circuit elec- 2,924,715 Patented Feb. 9, i960 2 trical signals which contain information concerning the intensity and the wavelength of the incident ultra-soft X-radiation. There is no partition or separating means between the two regions; hence, a common gas at a controlled pressure is present in both regions. As used hereinafter, the term X-ray optics region is defined to denote, broadly, any and all means within the first region for generating ultra-soft X-radiation and for utilizing and/or operating on said X-radiation for some particular purpose. For instance, these means may; include a source of X-radiation, such as an X-raytube or artificially-radioactive source. It may also include collimating means or split systems for shaping a beam of X-radiation fora particular purpose. Finally, if desired, it may include dispersive means such as a single crystal or grating for separating the radiation into its individual wavelengths. Furthermore, the term detection region denotes the second region in the enclosure in which X-radiation may be absorbedby an ionizable'gas medium therein, thereby to produce ion pairs which can be collected by a suitable electrode system and thus produce an electrical pulse in an output circuit. The apparatus of the invention utilizes the principle of controlled absorption and transmission of the ultra-soft X-radiation in the X-ray optics and detection-regionsso as to optimize the detection efficiency for a particular wavelength of the radiation. The manner by which this is accomplished will be explained in greater detail hereinafter.

In accordance with the method of the invention, the pressure of the gas atmosphere common to both the X- ray optics and detection regions is varied or controlled to maximize the sensitivity of the detection region for a particular X-radiation. By this technique, not only is the efiiciency of the detection of the impinging radiation improved,- but it is improved selectively, so that discrimination against unwanted radiation is achieved in the system of the invention, thereby simplifying the sepa: ration of wavelengths usually necessary to improve the accuracy of the system. To this end, means are provided coupled to the enclosure of the system for cha'ng ing the pressure of'the gas" therewithin. In addition, in accordance'with further features of the method of the invention, the ratio of X-ray optics path length to detection path length in the two regions" is" slowly varied While continuously detecting the incident radiation thereby to produce output information, which when properly uti. lized, enables the ultra-soft X-radiation t'o be directly analyzed.

The invention will now bedescribed in connection with the accompanying drawing, in which:

Fig. 1 illustrates, somewhat schematically, one form of apparatus accordihg to the invention for analyzing the principal fluorescent radiations of elements in the second period of the periodic table;

Fig. 2 is an elevational view of the apparatus shown in Fig. 1 along the line 22;

Fig. 3 is a graph showing counting efiiciency curves in the detection region" for the K-radiations of severalbele ments in the second period;

Fig. 4 illustrates a modification of the apparatus shown in Big 1, in which the ratio of X-ray optics to detection path lengths may be varied; I

Fig. 5 is a graph of data obtained by operating thesystem illustrated in Fig. 4 in a particular manner.

Referring now to the drawing, Fig. 1 shows one form of X+ray analysis system in accordance with the invention. The system shown comprises a housing or enclosure 5, which is substantially gas-tight and which is provided-with a gas-tightdoor' 6 afiording access to the interior. tube 7 for generating a beam of X-radiation of the proper energy content and of suificient intensity to excite Within the housing 5 is mounted an X-ray,

the elements constituting a specimen to be analyzed into pared by techniques well-known in this art, is mounted in a position to be irradiated by the X-radiation emanating from the tube 7, which irradiation excites the elements constituting the specimen 8 into generating their characteristic X-radiations in all directions therefrom. It is presumed for the purposes of this discussion that the specimen 8 contains one or more elements in the second period of the periodic table, which elements thus produce characteristic fluorescent X-radiation, i.e., their K-radiations, which are ultra-soft in character. For example, typical wavelengths for these radiations lie in the range of about 200 to 12 Angstroms. A single crystal 10, which may, for example, be of gypsum, is-mounted for rotation about an axis 11 and is positioned to receive some of the fluorescent X-radiation generated by the specimen 8. In the usual way, the single crystal 10 reflects or refracts selected wavelengths of that fluorescent X-radiation in particular directions relative to the surface thereof, as is shown by the dotted lines in the drawing. A slit system 12, shown schematically, is mounted for rotation about the same axis 11 as the single crystal 10. In addition, by means of suitable mechanical couplings, shown schematically as a pair of gears 13, the slit system 12 is caused to rotate about the axis 11 at twice the angular velocity as that of the crystal 10. United States Patent No. 2,449,066 describes a conventional system of this general type wherein this 2 to 1 rotational relationship between a crystal and a detector may be obtained. As is indicated in that patent, the angular position of the single crystal 10 and the slit system 12 relative to the specimen 8 determines which wave lengths of a plurality thereof present in the radiation generated at the specimen 8 will be passed on to the remainder of the system. This angular position may be indicated by a pointer and scale as shown at 15. A blocking or barrier member 16 is mounted between the X-ray tube 7 and specimen 8, and between the specimen 8 and the slit system 12, which blocking member serves to prevent direct radiation from the tube 7 from being incident on the crystal 10, and also prevents fluorescent X-radiation from the specimen 8 from passing directly through the slit system 12 without impinging first on the crystal 10. In accordance with the terminology employed hereinbefore, the elements described above are all located within the X-ray optics region of the system. r

These elments, it will be noted, include means for generating ultra-soft X-radiation, i.e., the specimen 8, and means for selecting portions of the X-radiation, e.g., monochromatizing or collimating the utra-soft X-radiation for various purposes, i.e., the elements 10 and 12.

The right half of the system shown in Fig. 1 comprises the detection region. The detection region comprises a cylindrical, slightly-curved, conducting enclosure 19 constituted by metal walls 20, 21, 22 and 23 of the housing, and a wire 24, for example, of 0.020 inch in diameter, extending through the center of said conducting enclosure 19 but insulated therefrom by means of an insulating head 25 sealed in the wall 22 of the housing. As shown more clearly in Fig. 2, entrance into the detection region is afforded by an elongated slit 26 located in the wall 21 of this conducting enclosure 19. The conducting enclosure 19 and the wire 24 constitute cathode and anode electrodes, respectively, of a Geiger- Mueller type of radiation detector or counter. To this end, .means 27 are provided for applying a relatively high potential, which can be varied, between the anode wire 24 and the cathode enclosure 19. In series with said potential source 27 is a resistor 28, across which electrical output signals may be den'ved in the usual way. The electrical signals, which may be in the form of pulses, are utilized in the normal manner, such as by counting them and recording the results. In order for counting action to obtain in the detection region, a suitable ionizable gas is provided within the housing 5. This is accomplished by means of a source or supply of gas 30, which is coupled by way of a suitable pressurecontrol means 31, such as a low-pressure regulator, to the housing 5. Also coupled to the housing 5 is a conventional form of pump 32 for first exhausting, if desired, the air within the housing 5, or for reducing the pressure of the gas within the housing 5 below atmospheric pressure. A pressure indicator 32 measures and indicates the gas pressure within the housing. It will be observed that the X-ray optics and detection regions freely communicate with one another through the unimpeded slit 26, and thus a common gas at the same pressure is present in both regions of the system. The cathode and anode electrodes in the detection region have the form illustrated to provide a large angular receiving area, so that for every position of the crystal 10 and slit system 12 within a limited angularrange, the reflected radiation will enter a sensitive and responsive volume of the detection region.

In operation, the ultra-soft, fluorescent X-radiation generated at the specimen 8 traverses an, optical path to the single crystal 10. I Certain Wavelengths of that fluorescent X-radiation are selected by the single crystal 10 and the slit system 12 and passed to the slit 26 constituting an entrance to the detection region. Inaddition, scattered X-radiation present everywhere in the X-ray optics region is also entering the detection region through the slit 26. The total opticalpath .for' the ultra-soft, fluorescent X-radiation in the X-ray. optics region is equal to about L as shown in the drawing. Q The radiation traversing that path enters the detection region through the slit 26. The path in the detection region traversed by the incident radiation is approximately equal to L Any radiation not absorbed by the gas present in the detection region is either lost in the. remote wall 20 or passes through that Wall 20 and out, of the system. To this end, the wall 20 may be constituted by some very thin, conductive material, such as beryllium, which has a retalively high transmission toX-radiation. Any ultrasoft X-radiation absorbed by the gas in the detection region will produce ion pairs, which will be attracted to and collected by the anode. and cathode electrodes therein, thereby to generate electricabsignals in the output circuit across the resistor 28. Q

However, in accordance with theinvention, the same gas present in thedetection region is also present in the .X-ray optics region; yet this gas in the X-ray optics region apepars to transmit the ultra-soft X-radiation to the direction region whereas the same gas in the detection region absorbs the same radiation to produce usableelectrical information. The reason for this apparent anomaly follows as. a direct consequence of ;one of the chief features of the invention, namely, to select a geometry for the X-ray optics'and detection regions at which certain desired radiations are transmitted to and absorbed in the detection region whereas other undesired radiations are absorbed either in the X-ray optics region or pass completely out ofthe system. Thus, in the con struction of the invention and one of its main advantages is what could be termed geometric discrimination against unwanted radiations. The following discussion will make this clearer.

All gases exhibit some absorption simultaneously with some transmission of X-radiation of all wavelengths. The relative ratio of absorption'to transmission of a particular path length of a gas depends solely on the wavelength of radiation involved, and the composition and pressure of the gas. In general, for a given path length, the absorption of the gas can be increased by increasing its pressure or by changing its composition to include a higher atomic number element. On the other hand, for

a gas of given composition and pressure, the absorption can be increased by increasing the path length in said gas traversed by the radiation. The invention utilizes these principles to provide geometric discrimination against unwanted radiation being absorbed and thus producing electrical information in the detection region. This is accomplished by providing an X-ray optics path of finite length and a detection region of finite length in a me determined relationship. When radiation comprising a plurality of soft and hard components, i.e., soft being longer wave-length radiation and hard being shorter wavelength radiation, traverses the finite X-ray optics path and the finite detection path, it is found that the absorption along this path is wavelength dependent. In particular, the softer wavelengths will be principally absorbed in the X-r-ay optics region and .thus produce only a very low counting efi'iciency in the detection region. The very hard radiation will be absorbed only slightly in the X-ray optics and detection regions, and thus pass essentially out of the system. Hence, for the set of conditions outlined above, it has been found that the absorption efficiency of the detection region for the intermediate wavelength radiation is the highest, whereas the absorption efficiency for the softer and harder radiations is lower. Thus, the electrical output information from the detection region which depends upon its relative absorption efiiciency comprises mainly information about the intermediate radiation and contains a correspondingly smaller quantity of information about the softer and harder radiations. In short, the geometry in thesystem of the inventionprovides selective absorption in the X-ray optics region to separate the wavelengths present in the generated ultra-soft X-radiation, thereby to provide information primarily about the wavelength desired.

From the foregoing, it will be evident that it is essential that L the length of the X-ray optics path, have a finite value. This stems from the fact that, for geometric discrimination to obtain, there must be some length of gas path before the detection region in which the softer radiations are principally absorbed, and no detection of which absorption occurs. In this connection, there is another important consideration. That is, that the absorption event that occurs when the gas absorbs the radiation, thereby producing ion pairs, is not limited to a single point in space, but actually covers a finite volume due to diffusion of the charges and scatter effects. It is necessary to define clearly the boundaries of the X-ray optics and detection regions in the sense that absorption events occurring in the X-ray optics region do not register as counts in the output circuit, which is coupled only to the detection region. It therefore follows that the X-ray optics region cannot be so small that this clear separation of the two regions is prevented. Thus, in the invention, it has been found that the X-ray optics path must be at least one cm. in length. Of course, if a-dispersing system is present in the X-ray optics region as illustrated in Fig. 1, a much larger X-ray optics path to accommodate the system is necessary. Further, in general, the detection region will be located as close as feasible'to the source of the ultra-soft X-radiation, and thus the upper limit of L will depend on the number and size of the elements present in the X-ray optics region.

In accordance with a further feature of the invention, the pressure of the gas within the housing and common to both the X-ray optics and detection regions is controlled, thereby to vary the selective absorption ofthe overall path and thus optimize the production of information from the detection region about a different wavelength of radiation. 'Ihat is to say, for a fixed geometry as illustrated in the drawing, it has been found that the counting efiiciency in the detection region will be a common housing 5 is reduced, which reduction results in a lower absorption of saidlonger wavelength radiation in the X-ray optics region and thus a shift of the maxi mum counting efliciency toward the longer wavelength region. At some pressure, it is found that the highest counting eificiency for this longer wavelength radiation now obtains in the detection region. Conversely, if it were desired to detect principally a shorter wavelength radiation, then it would be necessary to increase the gas pressure within the housing 5, which, similar to what was described above, would result in the maximum efliciency of counting in the detection region being shifted in the direction of the shorter wavelength radiation. For any particular geometry, or ratio of path lengths in the X-ray optics and detection regions, within certain limits as will be explained hereinafter, there exists a specific gas pressure at which a maximum counting efiicie'ncy in the detection region for a particular radiation will obtain. Thus, if the wavelength desired to be detected is known, it is necessary merely to select a particular pressure of the common gas within the enclosure, which will also be known, thereby to optimize the production of electrical information concerning that wavelength. This change in pressure can be readily obtained by means of the pressure control means 31 and pumpingmeans 32, together with the supply of gas 30.

This optimization of counting efliciency for a particular wavelength, at the expense of other wavelengths, matean enclosure containing a common gas, and having a first optics region of path length L in which ultra-soft X-radiation is generated and may be operated on or controlled, and a second detection region of path length L in which detection and counting of the ultra-soft X-radiation occurs. The relationshipbe'tween the counting efiiciency to a particular wavelength ofradiation in the detection region, the pressure and composition of the common gas, and the geometry of the system can be expressed mathematically in the following way:

where F is the fractional counting efiiciency of the X-rays that enter the system,

is the mass absorption coefiicient in the cm. /gm., p is the density of the common gas at standard conditions of room temperature and atmospheric pressure in gm./ cm. P is the pressure of the gas in the system in atmospheres or fractions of an atmosphere, Li is the optics path length in cm., and L is the detection path length in cm. Subscripts m and, d have been attached to the two Greek letters p to avoid confusion. Otherwise, they are the conventional symbols used in this art. From Equation 1, it can be shown that:

9 1M 1, L3 +ln(1 where F max is the maximum possible attainable etficiency of counting in the detection region and L is the overall path length and equal to L -+L The significant point derivable from Equation 2 is that the maximum counting 'region, and the length L of the detection region.

Equations 1 and 2 for maximum counting eflleieney yields I the following:

The factor on the left may be termed a control number, the product of the gas pressure and the detection path length. This control number is a function of the geometry of the system for a particular gas and wavelength, and denotes those combinations of pressures and detection path length at which the maximum counting efficiency for a particular radiation is realized. Otherwise stated, for every combination of pressures and path lengths which satisfy Equation 3, the system is tuned to maximum counting efiicieney for a particular radiation. However, other considerations dictate limits for these control numbers, and in accordance with the invention, only those systems which provide control numbers in the range of 0.01 to 400 are considered within the scope of the invention.

The factors which dominate the construction of a practicable system are the range of gas pressures in the enclosure which can be readily produced by commerciallyavailable'equipment, the length L of the X-ray optics For a practical system, the pressures of the common gas should lie in the range between about 0.01 and 2 atmospheres. In this range, detectors of the Geiger-Mueller type are readily available which operate with normal values of potentials, and are of reasonable size. Pressures below the lower limit are harder to maintain and less controllable; pressures above the two atmosphere limit render more difficult construction of a gas-tight enclosure. With ragard to L the length of the detection region, a feasible range of values lies between about 1 cm. and about 200 em. The minimum value is again dictated by reasons demanding a clear separation between the detection and X-ray optics regions. If L were made too small, then absorption events occurring at its border or even within its active region may result in some of the ion products being lost and thus not contributing to the formation of the electrical pulses. The maximum value is provided to minimize the eumbersomeness of the apparatus. As indicated before, the dimension of L will depend upon the elements present in the X-ray optics region. For the situation illustrated in Fig. 1, wherein a. specimen, a dispersing device and a slit system are mounted in the X-ray optics region, obviously the path length of the latter is considerable, for example, possibly 20 cm. or higher. On the other hand, the dimension of the X-ray optics path length in the embodiment illustrated in Fig. 4, which will be later described, is not as large. The radiations for which the system is tuned are, of course, the K-radiations of the elements in the second period of the Periodic Table. The only remaining unknown is a suitable gas with which the results of the invention may be achieved. By substitution in Equation 3, and in view of the other proseriptions on the system, it has been found that only a few counting gases are operable in the system of the invention. These are helium, hydrogen, nitrogen, neon, methane, and possibly some other low molecular weight gases. In short, only those gases which exhibit a rather low absorption to soft X-radiation. Helium is clearly the preferred gas, because it provides a fairly high counting effieiency for the ultra-soft X-radiations and within a readily-obtained range of pressures 'and for any number of practicable geometries. 'Several systems of the invention will now be described in detail.

I. For a counting eflicieney in the detection region of 50%, i.e., counting of 50% of the X-ray photons generated at the ultra-soft X-radiation source, the ratio of is 0.22. This ratio follows from Equation 2. For an overall path L equal to. cm., L =23.4 em. and L =6.6 em. With this geometry, tuning or peaking the detector response for the boron K-radiation occurs at a pressure of helium as the common gas of 0.04 atmosphere. Similarly, tuning for carbon occurs at a pressure of 0.105 atm.; for nitrogen, at a pressure of 0.277 atm.; for oxygen, at a pressure of 0.608 atm.; and for fluorine, at a pressure of 0.918 atm. Thus, with the dimensions indicated, a counting eflieiency of 50% of the ultra-soft X-radiations in the detection region is realized with a range of helium pressures of 0.04 to about 0.9 atmosphere. By way of passing, the control numbers for the foregoing example range from 0.943 to 21.23. It will be observed from this example that the length of the X-ray optics region is fairly small and may be inadequate for the provision of a dispersing system as illustrated in Fig. 1. This can be assisted by increasing the ratio of L to L though a sacrifice in counting efiiciency will result. Example II illustrates such a system.

II. For an overall optical path equal to 30 em. but with a ratio of L to L of 0.75, a maximum counting efficiency of 10% of the ultra-soft X-radiation in the detection region will be achieved. In this case, L =22.5 cm. and L =7.5 cm. To peak for the boron K-radiation, a helium gas pressure of 0.0236 atm. is employed; for carbon, 0.059 atm. is used; for nitrogen, 0.158 atm. is used; for oxygen, 0.355 atm. is used; and for fluorine, 0.53 atm. is used. For this example, the control numbers range between 0.1773 and 3.995.

III. For an optics path L equal to the detection path L and for an overall optical path equal to 13 cm., the maximum counting efficiency in the detection region will be 25%. Again, helium is the common gas employed in the enclosure.

Fig. 3 illustrates data obtained with a system as described in Example III. That figure shows a graph of fractional counting efiiciency in the detection region for certain wavelengths along the ordinant against the gas pressure of the helium along the absicissa. There are four curves in Fig. 3, representing, respectively, the counting efiiciency in the detection regio'n for this L =L =6.5 em. geometry of X-ray optics and detection path lengths for the principal fluorescent radiations, i.e., the K-radiations, from four elements in the second period of the periodic table. Curve represents the counting efliciency against pressure for the K-radiation of oxygen; curve 46, that for nitrogen; curve 47, that for carbon; and curve 48, that for boron. It will be observed from Fig. 3 that each of the curves exhibits a maximum point or peak for some particular pressure of the helium. In other words, by adjusting the helium pressure within the housing to a predetermined value, it is found that, for the geometry indicated above, the counting efiiciency in the detection region is a maximum for a particular wavelength of radiation. Moreover, the peaks of these elements 5 to 8, which are adjacent in the second period, are fairly widely spaced apart, so that at a particular pressure of the helium, the high counting effieiency corresponding to the selected wavelength contrasts markedly to the relatively lower counting efiieiency for the characteristic fluorescent radiation of an adjacent element in that same period. For example, at a helium pressure of 0.45 atmosphere, the counting efiicieney in the detection region for the K-radiation of nitrogen is 25%. Moreover, at that same pressure, the counting efficiency in the detection region for the K-radiation of oxygen will be only 19%, the counting efiiciency for the 'K-radiation of carbon 13%, and the counting efficiency for the K-radiation of boron practically zero. Thus, the information present in the electrical pulses appearing in the output circuit coupled to the detection region is representative primarily of the intensity or the K-radiations of nitrogen. The reasons'for thesernarked difierences in counting efficiency for'the K-radiations or adjacent elements inthe second period, which are also the chief reasons for the system being operable essentially only inthe ultra-soft and adjacent X-radiation regions; are the rather large differences in wavelength betweenthe K-radiations of these very low number atomic elements, 'andthe exceedingly large differences in value "of the mass absorption coeflicients for these wavelengths of'diflerent detecting gases. As eX- pl ained betore, the reduction in counting efliciency to the boron and carbon K-radiations at this selected helium pressure of 0.45 atmosphere is due to the fact that most of this softer radiation is being absorbed in the helium along the X-ray optics path, and hence very little of that radiation is reaching thedetection region. Conversely, with regard to the harder radiations produced by oxygen and higher numbered elements, these radiations are passed largely through bo'ththe X-ray optics and the detection regions and are lost either in the remote wall 20 or passed completely out of the housing. Thus, their relative quantity of absorption in the detection region is also low. At the selected helium pressure of 0.45 atmosphere, ho'wever, a relatively large quantity of the nitrogen K -radiation's are absorbed in the detection region, and his these absorption events which are productive of the majority of ion pairs collected by the anode and cathode electrodes in the detection region.

Itwill be noted that the effects described above cannot be obtained for the higher numbered atomic elements, which 'exist, s ay, in the fourth or higher periods of the periodic table. This is a consequence of the smaller relati differences in wavelength which exist between the adiations of adjacent elements at the higher atomic .numbegs and the smaller diiference in mass absorption coefiicients tor these wavelengths of difierent detecting 8 ,136

' I will h Qb v d i Q A QQfi With he gr p of F 3 p s of se s tiv ty o th detection gi q su a l e ium p essures. lying be ween ,at ph i about $4 of atmospheric. The corresponding control numbersfor L =6.5 cm, are .65 and 0.65. This would be preferred geometry, sincepressures of this order are relatively simple to, obtain and to control. However, from the foregoing, it will be evident that the invention is not limited to any specific geometry of X-ray optics and detector path lengths. On the contrary, many changes will be possible ,both in the overall length of the combined paths andthe ratio of the X-ray optics to. detection path length, within the limits prescribed earlier, without departing from. the principles of the invention. Moreoyer, only with certain detecting gases can the results desired be obtained. In general, the invention is operable only with those detecting gases. having linear absorption coefiicients at atmospheric pressure Which fall in the range of the corresponding coefficients of the ele ments neon and below in atomic number, and thus includes, a low weight organic detecting gas such as methane. The linear absorption coeflicient is the product of the mass absorption coeificient and the density p at atmospheric pressure. For example, if a gas having a higher linear absorption coefiicient, and thus a higher absorption, such as argon, were utilized, it would be impossible to construct a system with a finite L that could he tuned to .the K-radiations of the seco'nd period elements within the range of pressures prescribed above. Thus, for this latter construction, the control numbers, would lie outside the desired range.

' Fig. 4 illustrates another form of the system of the invention, which'is capable of being operated in a somewhat difierent fashion, -In particular, the system in Fig. 4 comprises" an enclosure 50, which is gas-tight, and which houses a source of ultra-soft Xrradiation 51. This source 51 may be constituted by a fluorescing specimen containing elements in the second period. It is assumed that source 51 produces ultra-soft X-radiation of at least two different wave-lengths. Mounted within the enclosure is a counter 53 defining a given detection region of finite length L The counter 53 comprises a hollow, metal cylinder 54, to which is connected at the top an insulated portion 55. Mounted on that insulating portion 55 is an anode wire 56, which extends along the axis of the cylinder '54. Coupled to the anode wire 56 and the cylinder 54 is an output circuit including a source of potential 57 and aresistor 58. The cylinder 54 constitutes a cathode electrode, which, together with the anode wire 5'6, constitutes a Geiger-Mueller type of detector, Whereby the production of ion pairs by a gas absorbing incident radiation will produce output pulses across the resistor 58 when the ion pairs are collected by the cathode and anode electrodes. Thus, the counting action in the enclosure 50 occurs within the detection region along an average path length of L The X-ray optics region, which is the region between the source of the ultra-soft X-radiation and the detection region, has, as shown in Fig. 4, an average pathlength equal to about L The rear of the cylinder 54 is open, so that radiation not abso'rbed within the region of the cylinder 54 will pass out of the detection region to an area in the enclosure where it will no longer be significant. Thequantity of absorption occurring along the overall path constituted by path lengths L and L depends upon the quality of the radiation emanating from the source 51. Thus, in a manner similar to that explained in connection with Fig. 1, the softer portions of the radiation will be principally absorbed in the "X-ray optics region, thereby producing a relatiyely low number of counts or pulses in the output circuit. Similarly, the harder radiations will pass through the X-ray optics and detection regions and again produce a relatively small number of counts in the output circuit. In the detection region, the counting efficiency of the intermediate radiations, on the other hand, is the highest andthus produces the greatest number of output pulses.

The system illustrated in Fig. 4 includes means for displacing the counter 53 along the overall path so as to enable the ratio of X-ray optics to detection path lengths Il /L to be varied as desired. These means, for example, may comprise a member 60 mounted for movement in a remote wall 61 of the enclosure 50 and including a rack portion 62 on its underside external to the enclosure. Coupled to that rack 62 is a pinion 63, which in turn is rotatable or actuable by a motor 64 of the usual form. Qoupled to the enclosure also is a supply of counting gas 66, for example, of helium. Via a pressure control device 67, a conventional pump 68 is also connected to the enclosure. By means of these three elements 66 to 68, it is possible to provide a given gas at a predetermined pressure within the enclosure 50 in a conventional manner. A pressure indicator 69 will enable that pressure to be measured and observed. Finally, means 70 are provided for coupling together, as desired, the motor 64 and the pressure control device 67. 'This will be explained in greater detail hereinafter.

The system illustrated in Fig. 4 is a non-dispersive system in which geometric discrimination is utilized to separate the wavelengths emanating from the ultra-soft X-radiation source 51. Hence, for a given ratio of optics to detection path lengths, a pressure of helium may be selected at which the system' is tuned to a particular wavelength of radiation,i.e., the sensitivity of the counter 53 to that particular wavelength is optimized, whereas its sensitivity to other wavelengths is reduced. The provision of the means for translating the counter 53 along the axis of the system, so that the ratio of X-ray optics to detection path lengths may-be varied, offers the additional advantage of optimizing within a given range of pressures the counting efficiency of the counter 53. For example, by selecting an X-ray optics to detection path length ratio, L /L of 1:1, with an overall combined path length of 13 cm., it is found that for a helium pressure of 0.15 atm., the counting efficiency in the detection region to the K-radiation of carbon is 25%. On the other hand, where the system permits, to attain a higher efliciency for this carbon K-radiation, it may be advisable to change the ratio of X-ray optics to detection path lengths, in addition to changing the pressure. Thus, by displacing the counter 53 to a position where L =2.85 cm. and L =10.15 cm., and by selecting a pressure of the helium equal to 0.237 atm., a counting efliciency in the detection region of 50% may be obtained. These figures derive from Equations 2 and 3. The coupling means 70 may provide these changes simultaneously. That is to say, coupling means of any conventional form may be provided at which the motor 64 displaces the counter 53 to a predetermined location with.

in the enclosure 50, and at the same time a predetermined pressure of the helium is established in the enclosure. The means for accomplishing this is considered obvious to those skilled in this art and further details thereof are not believed necessary.

The apparatus illustrated in Fig. 4 has the additional feature that it provides a very simple yet fairly accurate method of analyzing ultra-soft X-radiation containing a plurality of unknown wave'lengths. This method involves the following steps. The counter 53 is first located at the end of the'enclosure 50 remote from the source 51. Then, while the radiations traverse the optical path along the center of the enclosure 50, the counter 53 slowly scans spacewise along the optical path from the remote end to a position adjacent the source 51. In other words, the counter 53 is slowly translated from one end of the enclosure 50 to the opposite end. Meanwhile, the counter 53 continuously counts the ion pairs produced by absorption within the gas enveloped by the counter 53 during its movement along the optical path.

With the information thus obtained a graph (Fig. 5) can be plotted in which the logarithm of the counting rate is plotted versus the position of the counter 53 along the axis of the enclosure 50. This distance is referred to by the reference letter Z in Fig. 4, with Z decreasing as the counter 53 approaches the source 51. A curve 72 will result from the steps set forth above, if the source 51 is producing ultra-soft X-radiation of two different wave-lengths. The curve 72 results from the different quantities of absorption of the different radiations which occur along the combined path. By graphical means, one may obtain the slopes of the extremes of the curves 72, which are the slopes of the asymptotes. These are shown in the figure as dotted lines 73 and 74. The slopes of those lines are a direct measure of the energy of the radiation emanating from the source 51, and by means of ordinary calibration techniques well known in this art, the slopes can be correlated to specific energies. If a fluorescing specimen is the source 51, these energies are in turn then directly correlated to specific K-radiations of elements in the periodic table. The latter information, of course, identifies the elements producing the radiation at the source 51. The above method is characterized by its simplicity and the rapidity with which the identification can be made. It is not adapted for providing information concerning the proportions of the different elements in an unknown specimen, but it will quickly identify those elements.

.The techniques normally employed in utilizing the electrical information obtained in the systems illustrated in Figs. 1 and 4 are well known in this art of X-ray fluorescence spectrometry. It will be realized that the electrical information does not give an absolute indication, for instance, of the proportions of the elements in .512 the specimen. On the contrary, some 'form of calibration will be necessary. In its simplest form, the calibration will involve a comparison technique of the information obtained with the unknown specimen to the information obtained under identical conditions with a series of known specimens prepared in the same manner.

The systems described above and illustrated in Figs. 1 and '4 have been described in connection with helium as the common gas present in both the X-ray optics and detection regions of the system. It will be realized, however, that while helium is the preferred gas, the other gases listed hereinbefore are also suitable to produce the results desired of the invention. For example, hydrogen would be, from a radiation transmitting and absorbing standpoint, as convenient as helium; however, its chief drawback would be its potential explosiveness; thus extreme care would be necessary to handle same. In addition, for some purposes, the neon or nitrogen may be preferred, particularly where the gas can be utilized for its absorption edge to sharpen the separation of the wavelengths on opposite sides thereof.

It will further be appreciated that the counters constituting the detection region can be operated either as Geiger-type counters or as proportional counters, and in certain cases even as ionization chambers. The selection of a particular type of operation would depend upon the intensity of the radiation involved and the gas employed in the system. It will be appreciated that the Geiger-type of action provides the greatest amount of gas amplification and thus the largest amplitude signals in the output circuit. The proportional-counter-type of operation has the advantage of producing pulses whose amplitudes are energy dependent. The means by which a particular type of operation may be obtained is well known to those skilled in this art, and usually involves the application of a particular potential to the counter, which potential is dependent upon the gas pressure and the geometry of the counter. In the embodiment of Fig. 1, for example, where the inside diameter of the counter 19 is about 6.5 cm. (see' Example III), the wire diameter is 20 mils and pressures of the helium range between A of an atmosphere and 1 atmosphere, the potentials employed for Geiger-Mueller type of operation is about 2200 volts. For proportional counter action, a potential of 1800 volts suffices. Again, however, these figures are not critical and would certainly be different for different arrangements of the counter. It willalso be noted that the cylindrical-type of geometry of the counter illustrated in both Figs. 1 and 4 is also not essential to the invention, it being also possible to employ plate-like cathode and anode electrodes, or constructions employing a plurality of anodes and a plurality of cathodes, or, finally, in the general geometry illustrated in the figures, even a cathode which is not cylindrical in shape but, say, rectangular, or even spherical.

Further, in connection with the embodiment of Fig. 1, it may be noted that the source of potential 27 has been shown with an arrow therethrough to indicate adjustability. This may be necessary for the following reasons. In operating this system, the pressure of the helium within the enclosure will be varied so as to optimize the sensitivity of the detection region to a particular radiation. However, it may be found that a potential suitable for providing counting action in the detection region at atmospheric pressure may be far too high for the potential suitable for providing counting action at, say 5 of an atmosphere. For this reason, it may be necessary to adjust the potential applied to the counter for each different pressure employed within the enclosure, which can be effected automatically by coupling said potential source 27 to the pressure control means 31. As has been explained in connection with the operation of the system in Fig. 1, the absolute values of gas gain in the counting section and the absolute values of t sens iv ty are u sqntm led. i e 9pmtion of the system will" invglve the use of a's eris of calibration curves by which the electricalinformation obtainedfrom the unhnoyyn specimen willhe compared with that from a known specimensr' standard to obtain the informationdesired. "Thus; if theconditions under which the electrical output is obtained concerning the unknown specimen are identicalto' 1th conditionsunder Wlllchjlle calibration curves" from the own samples have been obtained, then the essential scientific relationships will be maintained and the information produced will provide an accurate analysis of the unknown.

It will thus be observed that the system and the method of the invention provide, for the first time known to us, a system for analyzing by means of gas detection the X-rays emitted by specimens containing unknown elements in the second period of the periodic table. The applications of an instrument constructed along the lines indicated are truly immense. It will make available to the art for the first time means by which a non-destructive analysis of an organic specimen may be made in a relatively rapid and simple manner, thereby avoiding the use of extremely cumbersome, time-consuming and expensive wet chemical methods. It is again emphasized that the results of the invention can be realized only when the apparatus described is employed in combination with ultra-soft X-radiation, because it is only with this particular range of X-radiations that the principle of geometric discrimination can be applied with a practicable apparatus.

While we have described our invention in connection with specific embodiments and applications, other modifications thereof will be readily apparent to those skilled in this art without departing from the spirit and scope of the invention as defined in the appended claims.

What is claimed is:

1. X-ray apparatus comprising an enclosure, means within said enclosure to generate ultra-soft X-radiation, means within said enclosure to transmit the X-radiation in a given direction, means within said enclosure positioned to intercept and detect said X-radiation, and an ionizable medium contained within and filling said enclosure,.said detection means comprising a pair of spaced electrodes defining a region in which said X-radiation is absorbed by said ionizable medium thereby generating electrical pulses.

2. X-ray apparatus comprising an enclosure, means Within said enclosure to generate ultra-soft Xradiation, means within said enclosure to transmit the X-radiation in a given direction, means within said enclosure posi tioned to intercept and detect said X-radiation, an ionizable gase contained Within and filling said enclosure, said detection means comprising a pair of spaced electrodes defining a region in which said X-radiation is absorbed by said ionizable gas thereby generating electrical pulses, and means to adjust the pressure of the gas within said enclosure to a value at which the sensitivity of said detection means is a maximum.

3. X-ray apparatus comprising an enclosure, means within said enclosure to generate ultra-soft X-radiation, means within said enclosure to transmit the X radiation in a given direction, means within said enclosure positioned to intercept and detect said X-radiation, an ionizable gas contained within and filling said enclosure, said detection means comprising a pair of spaced electrodes defining a region of given length in which said X-radiation is absorbed by said ionizable gas thereby generating electrical pulses, and means to adjust the pressure of the gas within said enclosure to a value at which the product thereof and said length is between about 0.01 and 400.

4. X-ray apparatus comprising an enclosure, means within said enclosure to generate ultra-soft X-radiation, means within said enclosure to transmit the X-radiation in a given direction, means within said enclosure positioned to intercept and detect said X-radiation, an ionizable gas contained within and filling said enclosure, said 14 detection means comprising a pair of spaced electrodes def ning a region in which said X-radiation is absorbed by said ionizable gas thereby generating electrical pulses, and means to adjust the pressure of said gasto a'value between about 0.01 and 2 atmospheres.

5. X-ray'apparatus comprising an enclosure, means within said enclosure to generate ultra-soft X-radiation, means within said enclosure to transmit .the X-radiation in a given direction, means within said enclosure positioned to intercept and detect said X-radiation, an ionizable gas contained Within and filling said enclosure, said detection means comprising a pair of spaced electrodes defining a region in which said X-radiation is absorbed by said ionizable gas thereby generating electrical pulses, means to adjust the pressure of the gas to a value at which a maximum portion of said X-radiation is absorbed in said detection region, and means to apply a potential to said electrodes.

6. X-ray apparatus comprising an enclosure, means within said enclosure to generate ultra-soft X-radiation, means within said enclosure to transmit the X-radiation in a given direction, means within said enclosure positioned to intercept and detect said X-radiation, and an ionizable gas having a linear absorption ooeflicient not greater than that of neon contained within and filling said enclosure, said detection means comprising a pair of spaced electrodes defining a region in which said X-radiation is absorbed by said ionizable gas thereby generating electrical pulses, and means to adjust the gas pressure within said enclosure to a value at which a maximum of said X-radiation is absorbed in said detecting region.

7. X-ray apparatus comprising an enclosure, means Within said enclosure to generate ultra-soft X-radiation, said means comprising a specimen adapted to generate ultra-soft X-radiation upon exposure to X-radiation con taining wave-lengths adapted to excite at least one element in said specimen to generate characteristic X-radiation, means within said enclosure to transmit the ultrasoft X-radiation in a given direction, means within said enclosure positioned to intercept and detect said ultrasoft X-radiation, an ionizable gas having a linear absorption coefficient not greater than that of neon contained within and filling said enclosure, said detection means comprising a pair of spaced electrodes defining a region in which said ultra-soft X-radiation is absorbed by said ionizable gas thereby generating electrical pulses, and means to adjust the gas pressure within said enclosure to a value at which a maximum of said ultra-soft X-radiation is absorbed in said detection region.

8. X-ray apparatus comprising an enclosure, means within said enclosure to generate ultra-soft X-radiation, means within said enclosure to transmit the X-radiation in a given direction, means within said enclosure positioned to intercept and detect said X-radiation, an ionizable gas consisting of helium contained within and filling said enclosure, said detection means comprising a pair of spaced electrodes defining a region in which said X-radiation is absorbed by said ionizable gas thereby generating electrical pulses, and means to adjust the value of the gas pressure to a value at which a maximum amount of X-radiation is absorbed in said detection region.

9. X-ray apparatus comprising an enclosure, means within said enclosure to generate ultra-soft X-radiation having a plurality of wave-lengths, means within said enclosure to transmit the X-radiation in a given direction, means within said enclosure positioned to intercept and detect said X-radiation, an ionizable gas contained within and filling said enclosure, said detection means comprising a pair of spaced electrodes defining a region'in which said X-radiation is absorbed by said ionizable gas thereby generating electrical pulses, and means to adjust the gas pressure within said enclosure to a value at which a maximum amount of X-radiation at one wave-length only is absorbed in said detection region.

10. X-ray apparatus comprising an enclosure, means within said enclosure to generate ultra-soft X-radiation nate against unwanted wave-lengths, and means to adjust having a plurality of wave-lengths, means within said enthe length of said absorbing region to further discriminate closure to transmit the X-radiation in a given direction, against unwanted wave-lengths.

means within said enclosure positioned to intercept and detect said X-radiation, an ionizable gas contained Within 5 References Cited in the file of this Patent and filling said enclosure, said detection means comprising a pair of spaced electrodes defining a region of given UNITED STATES PATENTS length in which said X-radiation is absorbed by said 2,516,672 Brockman July 25, 1950 ionizable gas thereby generating electrical pulses, means 2,602,142 Meloy July 1, 1952 to adjust the gas pressure within said housing to discrimi- 10 2,683,220 Gross July 6, 1954 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 2,924,715 February 9, 1960 Charles F. Hendee et al. It is hereby certified that error appears in the of the above numbered patent requiring correction and that the said Letters Patentjshould read as corrected below.

Column l line 53., for

apepars" read appears column 6, line 29, after "electrical" insert output column 13, line 52 for "gase" read gas Signed and sealed this 2nd day of August 1960.

(SEAL) lttest: KARL H, AXLINE ROBERT C. WATSON Lttesting Oflicer Commissioner of Patents 

